I. Field of the Invention
The present invention relates to diamond drag bits.
More particularly, this invention relates to diamond cutting elements for diamond drag bits.
II. Description of the Prior Art
Polycrystalline diamond compacts (PDC) have been effectively used for cutters on drag bits while drilling soft earthen formations in petroleum and mining exploration for more than a decade.
The most common cutter type used in PDC drag bits is classified in the drilling industry as a "stud" type PDC.
For example, a typical stud type PDC cutter is illustrated in FIG. 6 and FIG. 7 of U.S. Pat. No. 4,776,411 assigned to the same assignee as the present invention and hereby incorporated by reference.
Practically all stud-type PDC cutters used, to date, have been manufactured as two piece units. A thin layer (approximately 0.030" to 0.040") of polycrystalline diamond is chemically/metallurgically bonded to a face of a much thicker (approximately 0.150" to 0.190") right cylinder wafer of cobalt cemented tungsten carbide. This integral diamond/carbide compact is then brazed to a cobalt cemented tungsten carbide modified cylindrical stud or post at an angle of between 15.degree. to 20.degree. from the vertical axis of the stud. The top surface of the stud is typically radiused to conform to the diamond/carbide wafer cylindrical surface and relieved rearwardly normal to the diamond surface.
Although PDC stud type cutters, as currently manufactured, serve a very useful purpose, there are many disadvantages in their manufacture and application.
The flat on the stud to which the PDC wafer is brazed and the carbide side of the PDC wafer must have extremely fine ground surfaces to affect a braze of necessary strength. These grinding operations are time consuming and costly.
The bonding of the PDC wafer to the carbide stud is fraught with many variables that are difficult to control. The braze temperature is significantly higher than the thermal degradation temperature of the diamond table and the bond interface of the diamond and carbide. Therefore, the diamond has to be protected by a complicated heat sink apparatus that is difficult to control during the braze cycle. A high reject ratio is inherent in this process lowering output and driving up costs. The actual braze quality is difficult to determine even with the most sophisticated non-destructive testing equipment available. An undesirable level of less than good brazes go undetected and wind up as PDC cutter failures in the field. The brazing process can also cause incipient and premature failure of the bond of the diamond layer to the carbide wafer which also will show up as a PDC cutter failure in the field. It is also difficult to braze a PDC cutter wafter that has two or more carbide particle/diamond particle transition layers that have a high cobalt level because the high differential in thermal expansion causes the PDC layer to crack during the braze cycle.
A new stud type PDC cutter is disclosed that eliminates the need to braze a PDC wafer to a tungsten carbide stud, thereby obviating the problems and inadequacies described above in current PDC stud design and processes.